首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   332篇
  免费   27篇
  国内免费   7篇
测绘学   3篇
大气科学   27篇
地球物理   102篇
地质学   121篇
海洋学   26篇
天文学   53篇
综合类   3篇
自然地理   31篇
  2024年   3篇
  2023年   1篇
  2022年   9篇
  2021年   15篇
  2020年   14篇
  2019年   19篇
  2018年   17篇
  2017年   20篇
  2016年   18篇
  2015年   10篇
  2014年   18篇
  2013年   17篇
  2012年   18篇
  2011年   22篇
  2010年   18篇
  2009年   28篇
  2008年   18篇
  2007年   20篇
  2006年   13篇
  2005年   9篇
  2004年   10篇
  2003年   10篇
  2002年   7篇
  2001年   6篇
  2000年   2篇
  1999年   4篇
  1998年   3篇
  1997年   1篇
  1996年   2篇
  1993年   1篇
  1992年   2篇
  1989年   1篇
  1988年   1篇
  1985年   1篇
  1984年   2篇
  1983年   2篇
  1981年   1篇
  1980年   2篇
  1976年   1篇
排序方式: 共有366条查询结果,搜索用时 31 毫秒
121.
Annual cycles of convectively important atmospheric parameters have been computed for a variety of from the National Center for Atmospheric Research (NCAR)/National Centers for Environmental Prediction (NCEP) global reanalysis, using 7 years of reanalysis data. Regions in the central United States show stronger seasonality in combinations of thermodynamic parameters than found elsewhere in North America or Europe. As a result, there is a period of time in spring and early summer when climatological mean conditions are supportive of severe thunderstorms.The annual cycles help in understanding the large-scale processes that lead to the combination of atmospheric ingredients necessary for strong convection. This, in turn, lays groundwork for possible changes in distribution of the environments associated with possible global climate change.  相似文献   
122.
Aaron Kingsbury 《Geoforum》2006,37(4):596-609
Business associations (BAs) represent a widely prevalent institution contingent upon a plethora of regional and industrial contexts around the world. Although their role in local development is rarely highlighted this paper argues that analysis of the structure and strategies of BAs reveals important insights into the problematic nature of external economies, typically based on the willingness of highly independent small firms to share and possibly fund some common service(s). BAs illustrate cooperative behaviour and their fragmentation indicates limits to cooperation and thereby to the contested nature of this particular external economy. Conceptually, the paper interprets BAs as institutional expressions of local cooperation and theorizes their strategy and structure in terms of a ‘logic of exchange’ model. This model defines the relationships between BAs, their members (organizational domains) and governments in terms of the logics of ‘membership’ and ‘influence’ that help understand the opportunities afforded and tensions imposed by the rationale and dynamics of cooperation. Empirically, the paper examines the formation and performance of BAs in the restructuring of the Okanagan’s wine industry that was stimulated by the Free Trade Agreement (FTA) between the US and Canada in 1989 and the North American Free Trade Agreement (NAFTA) of 1991. In the Okanagan wine industry, a newly formed BA was vital in helping firms overcome the crisis generated by free trade. However, subsequent fragmentation that arose out of member disenchantment with logics of influence and membership, indicated limits to local cooperation that may constitute a significant diseconomy in a future crisis.  相似文献   
123.
Many marine fishes use spatially distinct habitats as juveniles and adults. Determining which juvenile habitats are most important to sustaining adult populations (i.e., which habitats are nurseries) has proven difficult, in part due to challenges in estimating survival of juveniles in putative nursery habitats. Recent technological advances have made largescale tagging efforts a viable approach to estimating survival of juvenile fishes by increasing recapture rates and enabling the use of individual-identification tags. These techniques, using Passive Integrated transponder (PIT) tags and autonomous antenna detection systems (antenna), have been successfully applied in freshwater environments. This paper reports the adaptation of these techniques to estuarine mangrove creeks (salinity: 2–28‰) for research of the juvenile life stage on an estuarine-dependent marine fish,Centropomus undecimalis. Retention rate of PIT tags in juveniles >120 mm standard length was 100%, with no mortality. The antenna detection field covered 48% of the creek water column, and the antenna was experimentally determined to detect approximately 67% of tagged fish that swam through. Overall recapture rate of tagged fish by the antenna was >40%. This recapture rate is higher than the sparse data typical of traditional tag-recapture studies. A time-dependent Jolly-Seber model was fit to the data, providing estimates of capture probability (0.8) and weekly apparent survival (0.41) that will be invaluable in comparing juvenile habitats of different quality (e.g., natural versus anthropogenically degraded). This research demonstrates the viability of this approach to fish research in estuarine habitats.  相似文献   
124.
A fast, automated, gas chromatographic system for the airborne measurement of PAN and a series of its homologues is described and its performance is evaluated. Response factors for PAN, PPN, APAN, PiBN, and MPAN have been determined and are discussed with regard to ECD response and to potential losses in the analytical system. Calibration methods used for these tasks are described and compared. The results from this work should help investigators who are employing the widely used GC/ECD method for the measurement of peroxyacyl nitrates to evaluate peaks of PAN homologues that cannot be calibrated for by using the reported response factors.  相似文献   
125.
126.
127.
Grass seed farmers have burned their fields in Idaho and Washington State for decades. Field burning, however, creates small particulate matter air pollution, thus engendering a growing public backlash by the 1990s that manifested itself in new clean air advocacy groups. The new groups’ push for policy change eventually met with significant success in both cases. How did each set of advocates approach the challenge of policy change? More specifically, what kinds of policy venues did each group choose and why? This research uses the cases to explore and explain each clean air group's choices vis-à-vis hypotheses of venue choice. Three hypotheses are tested—Schattschneider's (1960 Schattschneider , E. E. 1960 . The semisovereign people . New York , NY : Holt, Rinehart, and Winston . [Google Scholar]) “expanded scope of conflict” thesis, ACF's (Sabatier and Jenkins-Smith 1999 Sabatier , P. , and H. Jenkins-Smith . 1999 . The advocacy coalition framework: An assessment . In Theories of the policy process , ed. P. A. Sabatier , 117166 . Boulder , CO : Westview Press . [Google Scholar]) contention that groups strategically apply their resources in order to increase the likelihood of achieving their primary goal(s), and Pralle's (2003 Pralle , S. B. 2003 . Venue shopping, political strategy, and policy change . J. Public Policy 23 : 233260 .[Crossref] [Google Scholar], 2010 Pralle , S. B. 2010 . Shopping around: Environmental organizations and the search for policy venues . In Advocacy organizations and collective action , ed. A. Prakash and M. K. Gugerty , 177203 . Cambridge , MA : Cambridge University Press .[Crossref] [Google Scholar]) thesis that internal group constraints deter groups from moving into new venues.  相似文献   
128.
Genovesa Crater Lake is a remote, hypersaline lake in the northern Galápagos archipelago that contains a finely laminated sediment record. This sediment record has the potential to provide a high-resolution history of past climate variability in the eastern tropical Pacific. Here we present modern climate, lake, and sediment observations from 2009 to 2012 to explore how local climate variability influences Genovesa Crater Lake and its sediments. Surface lake temperature is strongly linked to air temperature and is highly seasonal. Temperature stratification is strongest during the warm season, whereas temperature becomes more uniform through the water column in the cool season. Deeper and earlier mixing occurred during the 2010 La Niña, which subsequently delayed 2011 cool season mixing and maximum warm season surface temperatures in 2011 and 2012. Lake salinity changes are influenced by precipitation, evaporation and persistent seawater influx. The largest declines in subsurface salinity follow months after the rainy season, when temperatures cool and fresher surface water from the previous warm/wet season mixes into the subsurface. Between 2009 and 2012, more calcium carbonate precipitated during a period of higher salinity. The period of highest calcium carbonate abundance measured in sediment records that span the late nineteenth to twentieth century coincides with the failure of two consecutive rainy seasons in 1988 and 1989 as well as the coldest monthly sea surface temperature measured at Puerto Ayora in 1989. More calcium carbonate-rich laminae from AD 1550 ± 70 to 1675 ± 90 may indicate a greater frequency of prolonged droughts or cooler temperatures, although enhanced productivity may also modulate carbonate precipitation. More Ca-rich laminae in Genovesa coincide with dry conditions inferred from other Galápagos sediment proxies, as well as prolonged dry and cool conditions inferred from reconstructions of the Southern Oscillation Index and NINO3 sea surface temperatures.  相似文献   
129.
We examine whether conservative white males are more likely than are other adults in the U.S. general public to endorse climate change denial. We draw theoretical and analytical guidance from the identity-protective cognition thesis explaining the white male effect and from recent political psychology scholarship documenting the heightened system-justification tendencies of political conservatives. We utilize public opinion data from ten Gallup surveys from 2001 to 2010, focusing specifically on five indicators of climate change denial. We find that conservative white males are significantly more likely than are other Americans to endorse denialist views on all five items, and that these differences are even greater for those conservative white males who self-report understanding global warming very well. Furthermore, the results of our multivariate logistic regression models reveal that the conservative white male effect remains significant when controlling for the direct effects of political ideology, race, and gender as well as the effects of nine control variables. We thus conclude that the unique views of conservative white males contribute significantly to the high level of climate change denial in the United States.  相似文献   
130.
Nearshore regions act as an interface between the terrestrial environment and deeper waters. As such, they play important roles in the dispersal of fluvial sediment and the transport of sand to and from the shoreline. This study focused on the nearshore of Poverty Bay, New Zealand, and the processes controlling the dispersal of sediment from the main source, the Waipaoa River. Hydrodynamics and sediment-transport in water shallower than 15 m were observed from April through mid-September 2006. This deployment afforded observations during 3-4 periods of elevated river discharge and 5 dry storms.Similar wind, river discharge, wave, current, and turbidity patterns were characterized during three of the wet storms. At the beginning of each event, winds blew shoreward, increasing wave heights to 2-3 m within Poverty Bay. As the cyclonic storms moved through the system the winds reversed direction and became seaward, reducing the local wave height and orbital velocity while river discharge remained elevated. At these times, high river discharge and relatively small waves enabled fluvially derived suspended sediment to deposit in shallow water. Altimetry measurements indicated that at least 7 cm was deposited at a 15 m deep site during a single discharge event. Turbidity and seabed observations showed this deposition to be removed, however, as large swell waves from the Southern Ocean triggered resuspension of the material within three weeks of deposition. Consequently, two periods of dispersal were associated with each discharge pulse, one coinciding with fluvial delivery, and a second driven by wave resuspension a few weeks later. These observations of nearfield sediment deposition contradict current hypotheses of very limited sediment deposition in shallow water offshore of small mountainous rivers when floods and high-energy, large wave and fast current, oceanic conditions coincide.Consistently shoreward near-bed currents, observed along the 10 m isobath of Poverty Bay, were attributed to a combination of estuarine circulation, Stokes drift, and wind driven upwelling. Velocities measured at the 15 m isobath, however, were directed more alongshore and diverged from those at the 10 m isobath. The divergence in the currents observed at the 10 and 15 m locations seemed to facilitate segregation of coarse and fine sediment, with sand transported near-bed toward the beach, while suspended silts and clays were exported to deeper water.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号